1. Field of the Invention
This invention relates generally to methods of creating scores in thermoplastic film and more specifically to methods of creating scores in thermoplastic film using ultrasonic energy.
2. Description of the Prior Art
Plastic pouches are used in various industries to contain many types of materials. In the medical industry, plastic pouches are frequently used to contain medical instruments, and other devices necessary for various procedures. It often is necessary to sterilize the materials in such pouches after the pouches have been filled and sealed. Therefore, it is desirable to use materials to form the pouch which can be subjected to various methods of sterilization, such as radiation, steam, or ETO sterilization techniques. Since it is often desirable to sterilize materials within the pouch, not all materials commonly used to form pouches can be selected for medical applications. One of the preferred class of materials used to form pouches for medical applications is the class of thermoplastic films.
Thermoplastic films are desirable to use because they can be subjected to ETO and radiation sterilization. They are also desirable to use because they are generally very durable and provide a biological barrier to protect any instruments after sterilization. Another advantage of using thermoplastic materials to form pouches for medical applications is that the pouches can be made using clear thermoplastic films which allow medical personnel to easily see the contents of a pouch.
As noted above, thermoplastic pouches are generally very durable. Therefore, a means must be provided in the pouch to allow medical personnel to easily open the pouch. Generally, a score line, or line of weakness, is provided across a pouch to provide a means of enabling medical personnel to easily open a pouch by pulling on the pouch on opposite sides of the score line. It is important that the score line sufficiently weaken the pouch to allow easy opening but not to allow inadvertent breakage or tearing of the pouch during shipping or handling. Therefore, it is important that the score line have a highly predictable strength value that is lower than the strength value of the rest of the pouch and that can be consistently reproduced during the manufacturing process of the pouch. If it is possible to produce a score line with a highly predictable strength value, then the rate of scrap during the manufacturing process is greatly reduced. The possibility of inadvertent opening of a pouch after it has been filled, sealed and sterilized is also greatly reduced, thus reducing long-term medical costs.
Thermoplastic pouches having score lines have been produced in the past using various manufacturing techniques and materials. One commonly used technique is to interrupt the flow of a molten thermoplastic material as it exits an extrusion die. This can be accomplished by inserting a probe across the path of the molten material. After the material cools and hardens, the interruption of flow results in an area of weakness in the resulting thermoplastic film. This method of interrupting the flow of molten material from a die can be used in either blown-film or flat-sheet-extrusion manufacturing processes.
Another technique for producing a score in a pouch commonly used with laminated materials is to apply directed radiant energy onto the thermoplastic film to vaporize a portion of the laminate to produce an area of decreased thickness. This process is described in U.S. Pat. No. 3,909,582 to Bowen.
Another technique for producing scores in pouches is to pass the material forming the pouch through matched embossing rollers which deform a selected portion of the pouch to produce a score having a desired depth.
While each of the techniques of producing score lines described above has value in various applications, it has been found that generally speaking, such techniques do not produce the desired accuracy and predictability required for certain medical applications. In particular, if a thermoplastic film is used that is very durable, it can sometimes be difficult to create a score line of sufficient yet predictable weakness using the techniques described above. Also, many of the techniques described above are limited in that the rate at which material can be scored can limit the overall speed at which pouches can be produced. Therefore, a need existed to develop a method of producing score lines in highly durable plastic film which is predictable and does not limit the rate at which film can be produced.